These studies will examine the fetal and placental metabolism of two amino acids: 1) an essential amino acid, threonine and 2) alanine. Both are potential glucose precursors. The studies will compare their rates of supply to the fetus and placenta with the fetal and placental requirements. The studies will be carried out in chronically catheterized pregnant sheep, studied under steady state unstressed conditions. Multiple tracer methodology will be used to study their metabolism in fetuses in which placental, fetal and fetal hepatic metabolism can be separated. The oxidation rate, plasma disposal rates and net hepatic uptake and metabolism of these amino acids will be studied under normal conditions and in two different models associated with intrauterine growth retardation, one associated with reduced glucose delivery to the fetus and glucogenesis and the other with normal glucose delivery. Net tracer and tracee fluxes will be measured into and out of the fetus, the fetal liver and the uteroplacenta. Carbon labelling of potential products of threonine and alanine metabolism will also be determined, particularly for other amino acids, glucose and lactate.